Macbook pro 07 - cannot burn double layer

My 2007 macbook pro cannot burn double layer DVDs (+R DL) although the system profiler says it should be able to do it. Is this a software or hardware problem? That is, should I buy e.g. Toast (and use it with the internal DVD burner) or an external disc burner (and use Disk Utility)?
Thanks

MATSHITA DVD-R UJ-857D:

Firmware Revision: KCVB
Interconnect: ATAPI
Burn Support: Yes (Apple Shipping Drive)
Cache: 2048 KB
Reads DVD: Yes
CD-Write: -R, -RW
DVD-Write: -R, -RW, +R, +R DL, +RW
Write Strategies: CD-TAO, CD-SAO, DVD-DAO
Media: To show the available burn speeds, insert a disc and choose View > Refresh

Just in case, here is the general info:
Hardware Overview:

Model Name: MacBook Pro
Model Identifier: MacBookPro2,2
Processor Name: Intel Core 2 Duo
Processor Speed: 2.16 GHz
Number Of Processors: 1
Total Number Of Cores: 2
L2 Cache: 4 MB
Memory: 1 GB

Burning double layer media seems to be a hit or miss thing. Some folks have complained that no matter what they tried (different media brands, various software, slower burn speeds, etc) that it just didn't work. While others have reported success.

Purchasing Toast or an external burner to burn double layer media is a decision only you can make. Both are going to cost $$ and there's no guaranty that you'll have success. Just my opinion.

Regards.

So it is not clear if it is a software or hardware problem...
(I want to buy one but not both if not needed. If it is Disk Utility's fault, I will buy Toast and keep using the internal dvd burner. If it is the burner's fault, I will buy an external burner and use Disk Utility.)
Thanks

That is the problem solving what is not working however if it helps I use Toast Titanium and burn DL no problem. Just set to burn at 'best' speed.

Thanks, I think I will give Toast a try.